The invention lies in the field of nuclear power generation technology. More specifically, the invention relates to a container for collecting and spreading core melt in a nuclear power plant with a wall part and a bottom. The invention further pertains to a nuclear power plant with a reactor pressure vessel which is disposed in a reactor cavity and contains a reactor core, and with a container for the collection and spreading of core melt.
The design of nuclear power plants, especially light-water nuclear reactors, must provide safety provisions for the control of extremely unlikely accidents in which the reactor core melts down. This is accomplished, for example, by providing a space immediately underneath or below and laterally adjacent of the reactor pressure vessel in which core melt formed by a melting reactor core can be collected and cooled. It is important, thereby, to avoid pollution of the surroundings. The effects of an unlikely accident of this kind thus remain confined to the nuclear power plant. In the case of a lateral disposition, the collecting space is of large-area design, as described, for example, in the international PCT publication WO 94/29876. That core catcher allows the core melt to spread out thinly over a large area, thereby forming a large surface area which can be cooled in an effective manner. A passage leads underneath the reactor pressure vessel from the reactor cavity in which the reactor pressure vessel is accommodated to the spreading space. The passage is closed during normal operation of the nuclear power plant. The spreading space and the reactor cavity are free from water during normal operation. The possibility of a steam explosion is thus reliably avoided in the event of the emergence of core melt. Only when the core melt enters the spreading space is water introduced passively into the spreading space. Steam which forms during this process can be discharged from the spreading space over a large area. In all cases, it is ensured that uncontrolled steam formation in the reactor cavity does not occur.
German published patent application DE 43 22 107 A1 has disclosed a structure for collecting and cooling core melt, in which a coolant flows through cooling passages which are arranged in the bottom of a core melt spreading chamber. As a result, only the bottom is cooled.
It is accordingly an object of the invention to provide a collecting container for core melt in a nuclear power plant, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensures even more effective and passive cooling of the core melt with at most little and controlled formation of steam. It is a further object of the invention to provide a nuclear power plant with a collecting container for core melt in which the formation of steam and hydrogen in the event of the emergence of core melt is largely prevented.
With the foregoing and other objects in view there is provided, in accordance with the invention, a container for collecting and spreading core melt in a nuclear power plant, comprising:
a structured bottom
formed of a material of good thermal conductivity; and
formed with a plurality of geodetically lowest points
and a plurality of geodetically highest points;
an outer wall extending with an upward slope between a respective one of the geodetically lowest points and a respectively adjacent one of the geodetically highest points; and
a steam conduit extending through an interior of the container and at each of the geodetically highest points.
In accordance with an added feature of the invention, the bottom has an inner wall extending with an upward slope between a respective one of the geodetically lowest points and a respectively adjacent one of the geodetically highest points.
The term xe2x80x9cgeodetically highest pointsxe2x80x9d as used herein is to be understood as describing the local maxima of the bottom, or floor. They can lie in one-plane or at different geodesic heights. The geodetically lowest points are local minima of the bottom and can lie in one plane or at geodetically different levels. An upward slope of the outer wall of the bottom between a geodetically highest point and an adjacent geodetically lowest point means that a horizontal profile between these two points, i.e. a profile at a constant geodesic level, is largely excluded. This has the advantage that cooling liquid in contact with the outside of the rising or falling bottom rises along the bottom due to heating, and steam bubbles which form in it can be discharged via the steam conduit. These are designed such that a two-phase mixture (steam/cooling liquid) can flow through in a sufficient quantity even in a transient flow phase. They are composed, for example, of a thermally insulating material or are surrounded by such a material. The upward slope is an effective means of preventing the formation of a steam-bubble region on the outside of the bottom, leading to impairment of heat transfer out of the container to the cooling liquid. It is particularly advantageous that the core melt flowing into the container does not come into direct contact with a cooling liquid, thus reliably preventing the uncontrolled formation of steam and a steam explosion. All that occurs is heat transfer between the core melt and a cooling liquid surrounding the bottom or a cooling device connected to the container. By avoiding direct contact with metal which may flow into the container after the core melt, a hydrogen-forming reaction between a cooling liquid, especially water, and this metal is additionally avoided. The bottom preferably extends between a geodetically lowest point and an adjacent geodetically highest point with a strictly monotonic slope, it being possible for the slope between a geodetically lowest point and a geodetically highest point to be constant. This makes the bottom particularly simple to produce from the point of view of manufacturing technology. The bottom preferably has a wall thickness of 1 cm to 10 cm, in particular 5 cm, this wall thickness being used for preference in particular in the case of a bottom where the thermally conductive material is a steel. Since the bottom comes into direct contact with the core melt, the thermal conductivity of the material for the bottom should be chosen in such a way that a crust forms as rapidly as possible in the region of the bottom. The thermal conductivity of a steel is so high that solidification of the core melt in the immediate vicinity of the bottom can be achieved within a few minutes. The container is preferably closed by a top made of a material of good thermal conductivity. Although the top does not come into direct contact with the core melt when core melt enters the container, a high thermal load is imposed on the top due to heat radiation. The top is therefore preferably to be designed in such a way that the heat given off by heat radiation can be rapidly dissipated. The top can be connected to a separate cooling system or, in particular, can be connected by way of the steam conduits to a cooling liquid surrounding the bottom.
In accordance with an added feature of the invention, a plurality of conduits pass through the top and a differential-pressure-dependent shut-off element closes the conduits for a fluid flow. This avoids the formation of excess pressure in the container. The shut-off element can, for example, be a one-way bursting disc. The bursting disc is preferably triggered only at a differential pressure at which the pressure within the container is higher than outside the container. For this purpose, the bursting disc is, for example, supported in the direction of the container by a metallic undernet. The conduits which pass through the top are taken to a geodesic level such that they project beyond the level of a cooling liquid serving to cool the top.
In accordance with an additional feature of the invention, the bottom is formed in a dome shape or a pyramid shape around a respective geodetically highest point. In other words, the bottom is formed with singular geodetically highest points, which represent the tips of respective pyramids, cones or half-shells (domes). The domes or pyramids (cones) have a circular or rectangular cross-section. It will be understood that other cross-sectional shapes, for example triangular or oval, are likewise possible.
In this case, the bottom of the container has a grid-like structure, the geodetically lowest points lying on grid lines and essentially singular geodetically highest points being arranged in the triangles quadri-laterals formed by the grid lines. It is a particularly simple matter to connect a respective steam conduit at the singular geodetically highest points. The geodetically highest points and the geodetically lowest points can likewise be arranged on non-intersecting lines, the bottom thus being structured with non-intersecting, essentially parallel furrows. It is also possible for the intersecting grid lines to be formed by the geodetically lowest points.
In accordance with another feature of the invention, the bottom has a wall with a thickness of between 1 cm and 10 cm. In a preferred embodiment, the bottom material with the good thermal conductivity is metal.
With the above and other objects in view there is also provided, in accordance with the invention, a nuclear power plant of the type having a reactor pressure vessel disposed in a reactor cavity and enclosing a reactor core, with a container for collecting and spreading core melt escaping from the reactor cavity upon a meltdown accident. The container comprises a wall part and a bottom part contiguous with the wall part, the bottom part:
being formed of a material of good thermal conductivity;
being formed with a plurality of geodetically lowest points and a plurality of geodetically highest points; and
including an outer wall extending with an upward slope between a respective one of the geodetically lowest points and a respectively adjacent one of the geodetically highest points, and being selectively surrounded by a cooling liquid;
a steam conduit extending through an interior of the container and at each of the geodetically highest points.
A nuclear power plant of this kind with a container for the collection and spreading of core melt, the bottom of which slopes upwards and downwards alternately and can be cooled externally by means of a coolant ensures that the core melt which has been collected and retained on the bottom in the container is cooled effectively without the formation of a steam film of poor thermal conductivity on the outer wall of A the bottom. The avoidance of such a steam film ensures effective cooling and reliable enclosure of the core melt. In particular, because of the good thermal conductivity of the bottom, rapid formation of a crust on the melt occurs in the region of the bottom. Any steam which does form on the outer wall is removed in an effective manner by way of the steam conduits, for which purpose these are in each case provided at a geodetically highest point of the bottom, to which the steam which forms rises. The bottom is preferably made from a metal, in particular a steel. The wall part of the container can likewise be made of-a material of good thermal conductivity, a metal. However, it can also be integrated into a supporting structure of the nuclear power plant, in particular a concrete structure. As an alternative, the steam conduit can be produced from a thermally conductive or thermally insulating material. By means of external cooling of the container without direct contact between the core melt and cooling liquid, both a steam explosion and the formation of hydrogen, in particular by the molten metal from the nuclear power plant carried along in the core melt, is reliably avoided. The formation of radioactive aerosols at the surface of the core melt by the avoidance of direct contact with the cooling liquid is also significantly reduced. If aerosols are formed, their spread is furthermore restricted to a small spatial area. In particular, no aerosols are released to the surroundings of the nuclear power plant.
The container is preferably closed by a top made of a thermally conductive material which is capable of absorbing the thermal radiant power released by a core melt and of releasing it to a cooling system or directly to a cooling liquid. This ensures that both sufficient thermal energy is dissipated via the bottom, which is in direct contact with the core melt, and that the heat released from the melt by thermal radiation is absorbed quickly at the top.
In accordance with a further feature of the invention, in case of an accident involving a reactor core meltdown, the container is completely surrounded by a cooling liquid In a further embodiment, the container is also completely surrounded by the coolant during a normal operation of the nuclear power plant. In the preferred embodiment, the container is completely surrounded by the cooling liquid, possibly even during normal operation of the nuclear power plant or it may be supplied by flooding when there is an accident involving a reactor core meltdown. In both cases, it is ensured with certainty that when the core melt enters the container in the case of an accident involving a reactor core meltdown, the container is completely surrounded by a cooling liquid for the purpose of cooling.
In accordance with again another feature of the invention, there is provided a supporting structure supporting the container. The container is fixed to the supporting structure so as to allow a relative movement thereof upon a change in temperature. Depending on the dimensioning and choice of materials, the container can be fixed to a supporting structure of the nuclear power plant from above, in particular suspended from it, or be supported or anchored from below. The container is preferably fixed in such away that it can move with changes in temperature, thereby reducing the occurrence of thermal stresses when the core melt flows in. The container is preferably connected at a point geodetically below the reactor pressure vessel to an opening of the reactor cavity, for example by means of a passage, a chute or similar means.
In accordance with a concomitant feature of the invention, the container has a horizontal cross-sectional area of between 100 m2 and 200 m2, in particular 170 m2. This leads to effective spreading of the core melt over a large area. By virtue of the rising and falling profile of the bottom of the container, the surface area of the bottom which is actually available for cooling by direct contact with the core melt is significantly larger than the horizontal cross-sectional area of the bottom. It is thus possible to determine the cooling performance of the container and the spreading of the core melt in accordance with requirements by the choice of the structuring of the bottom.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in container for the collection and spreading of core melt and a nuclear power plant with such a container, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.